A Software-Defined Platform for RF Test

While engineers have long used LabVIEW as a programming language to automate instrumentation, new communications toolkits in LabVIEW enable engineers to test many RF devices using virtual instrumentation.  Unlike traditional RF signal generators and analyzers that contain fixed personalities for standards such as GPS, Wireless LAN (WLAN), Fixed WiMAX, and others, virtual instrumentation provides equivalent measurement functionality while maintaining an “open-source” software architecture.  The open source toolkit approach to software-defined RF test produces many benefits including:

• Measurements for a variety of wireless standards including WLAN, WiMAX, GPS, and others
• Integrated DC, digital, and baseband measurements onto same system
• Significantly faster measurement times with delivered by multicore central processing units (CPUs)

LabVIEW toolkits for WiMAX, Wireless LAN (WLAN), and GPS utilize virtual instrument handles to create baseband waveforms, apply IQ impairments, perform measurement, and set RF characteristics.  While RF toolkits and the corresponding example programs are explicitly designed to operate with PXI RF instrumentation, they can also be used with other instrumentation and to simulate physical layer RF characteristics.  This tutorial examines the architecture of RF LabVIEW toolkits and the ways you can use them to simulate physical layer impairments in software.

NI GPS Simulation Toolkit

With the NI GPS Simulation Toolkit for LabVIEW, you can create continuous GPS baseband waveforms that are up to 24 hours in duration. This toolkit is designed to translate satellite information contained in almanac and ephemeris files into course acquisition (C/A) GPS waveforms.  The flexible API enables you to customize signal settings such as simulated latitude/longitude/altitude and receiver velocity. You can even create custom motion trajectories to simulate the movement of a GPS receiver. Internally, the toolkit applies appropriate Doppler shifts to each satellite, and the device under test (DUT) behaves as it is moving according to the prescribed path.  Figure 1 illustrates how to configure a generation session to generate in script mode.

 

Figure 1. Generating motion trajectory scripts with the GPS Toolkit

Using script mode, you can enter either a series of GPS waypoints or a series of “straight” and “arc” commands.  For example, the following script in Table 1 configures a generation session that simulates a receiver moving on an oval track.

Table 1. GPS receiver motion trajectory

With features such as custom motion trajectories, engineers can use the GPS toolkit to create baseband waveforms that simulate custom receiver movement.  The toolkit provides LabVIEW example code to create GPS waveforms that can be either written to a binary file or generated with a PXI RF vector signal generator.

Creating WLAN Waveforms in LabVIEW

The NI WLAN Measurement Suite provides generation and analysis functions for IEEE 802.11a/b/g physical layer measurements. The suite includes the NI WLAN Analysis Toolkit for WLAN measurements and the NI WLAN Generation Toolkit for signal creation.  To simulate transmitter or receiver impairments, you set WLAN impairment properties through a property node.  While general purpose NI LabVIEW toolkits such as the NI LabVIEW Modulation Toolkit can create general-purpose modulated signals such as AM, FM, FSK, QPSK, 64-QAM and others, specialized RF toolkits use a virtual instrument handle to create standards-based baseband waveforms. Figure 2 illustrates the block diagram of a LabVIEW example used to create a WLAN baseband waveform.

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Figure 2. Creation of WLAN Baseband

As Figure 2 illustrates, you can use the NI WLAN Generation Toolkit to create highly customized signals. Here, a property node is used to set characteristics such as DC offset, IQ gain imbalance, and quadrature skew. By setting these characteristics, you can simulate the quadrature impairments present on a direct quadrature modulator. In Figure 3, observe the constellation plot of a 16-QAM (24 Mbits/s 802.11g) signal with 1 dB of IQ gain imbalance and 1 deg of quadrature skew.

Figure 3. Analysis of impaired WLAN Signal

In figure 3, the constellation plot has been produced by the WLAN Analysis Toolkit without the use of actual measurement hardware.  Here the measured error vector magnitude (EVM) is -21.5 dB.

Simulated Fixed WiMAX Waveforms in LabVIEW

Similar to WLAN, the Measurement Suite for Fixed WiMAX offers LabVIEW programming APIs for signal generation and analysis of Fixed WiMAX Signals.  Waveforms are generated according to the IEEE 802.16-2004 (also referred to as IEEE 802.16d) physical (PHY) layer specifications.  In addition, the toolkit supports software-defined impairments.  One common impairment that is native to the NI Generation Toolkit for Fixed WiMAX is additive white Gaussian Noise (AWGN).  You can use this impairment to simulate broadband noise – either from a physical channel – or from analog-to-digital converter (ADC) and digital-to-analog converter (DAC) quantization noise.  To add AWGN to a simulated Fixed WiMAX waveform, you must programmatically set two properties: “AWGN enabled” and “SNR” (for signal-to-noise ratio).

Figure 4. Simulated AWGN can be added with a property node.

For simulated WiMAX waveforms, the measured EVM of the modulated signal is nearly identical to the level of simulated AWGN.  For example, when we simulate an SNR of 31 dB, the EVM measured by the NI Analysis Toolkit for Fixed WiMAX reports a result of -31.2 dB.  Figure 5 shows a constellation plot of the simulated sub-frame.

Figure 5. Constellation of Fixed WiMAX Subframe

The smeared or blurred constellation points in Figure 5 indicate that the simulated waveform contains impairments.  Note that the NI Signal Generation Toolkit for Fixed WiMAX enables you to create customized frames and sub-frames.  Consistent with the IEEE 802.16-2004 standard, a simulated sub-frame can contain multiple bursts.  In this example, our the simulated waveform contains a 64-QAM burst (green), a 16-QAM burst (black), a QPSK burst (red), and a BPSK burst (blue).

Communications System Design in LabVIEW

In addition to native wireless standards toolkits, LabVIEW contains a wide variety of functions and algorithms that are ideal for communications system design.  Moreover, the NI modulation toolkit contains functions for modulation and demodulation, upconversion and downconversion, and simulation of impairments such as multipath fading, blocking signals, and even phase noise.  As an example, Figure 6 shows the block diagram of a VI that resamples a baseband IQ waveform, adds multi-tone blockers, and upconverts to a digital IF.  Note that the fractional resampler is set to 200 MS/s in Figure 6, but can be used for a nearly unlimited range of fractional sample rates.

Figure 6. Upconversion of simulated WLAN waveforms

With functions for upconversion and downconversion to and from IF/RF, you can simulate channel impairments at both baseband and RF/IF.  The block diagram in figure 6 illustrates how to apply a multi-tone blocker signal to a baseband waveform.  The power spectrum of the simulated output is shown in Figure 7.

Figure 7. Power Spectrum of Simulated Fixed WiMAX Waveform at IF.

In addition to applying impairments with specialized communications toolkits such as the modulation toolkit, you can also apply many other signal processing routines with core LabVIEW functions.  In figure 8, observe the block diagram of an example that applies FIR and IIR lowpass filters to a simulated baseband waveform – and then performs an EVM measurement on the filtered baseband signal.

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Figure 8. LabVIEW digital filters applied to a simulated WLAN waveform

When you digitally filter a simulated WLAN waveform that contains AWGN, you can effectively remove broadband noise that it outside of the bandwidth of interest.  When we execute the code shown in Figure 8, we observe that applying an elliptic IIR filter improves the measured EVM.  Figure 9 shows a zoomed in constellation diagram of the filtered and unfiltered baseband symbol location.  In this case, the simulated WLAN waveform has an SNR of 37 dB. 

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Figure 9. Constellation of filtered and unfiltered baseband

In Figures 8 and 9, the specific IIR filter implemented was a 5^th order lowpass elliptic filter with a 10 MHz bandwidth, a passband ripple of 0.1 dB, and a stopband attenuation of 65 dB.  Figure 9 illustrates the result of a lowpass filter on an OFDM demodulator.  By filtering broadband noise, the EVM result improved from -37.0 dB to -43.6 dB.

Using RF Toolkits with Mixed Signal Instrumentation

One of the biggest benefits of RF toolkits for GPS, WLAN, and WiMAX waveforms is that simulated waveforms can be generated with a wide variety of PXI modular instrumentation.  For example, one can use a high-speed digital pattern generator (HSDIO) to generate modulated waveforms as digital baseband.  To test a quadrature modulator, a dual-channel differential arbitrary waveform generator (like the NI PXIe-5450) can be used to generate modulated waveforms as analog baseband.  Finally, you can use RF vector signal generator (such as the PXIe-5673) to generate these waveforms at RF frequencies.  Figure 10 illustrates a basic WiMAX generation example that produces a Fixed WiMAX signal in a variety of different waveform types. 

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Figure 10. Creating analog and digit WiMAX baseband waveform

In figure 10, we observe that while the NI Generation Toolkit for Fixed WiMAX produces baseband signals as a complex waveform (for RF vector signal generators), this same waveform can be converted to analog baseband I/Q (for a dual channel AWG) or to digital baseband I/Q (for a digital pattern generator).

Conversely, analysis toolkits such as those for WLAN and WIMAX can also be used with a wide variety of input devices as well.  Using either Modulation Toolkit or core LabVIEW functions, you can convert digital baseband, analog baseband, and even intermediate frequency signals to the appropriate format for the RF analysis toolkits.

RF Toolkits in LabVIEW

Wireless standards toolkits in LabVIEW provide a fully flexible solution to both design and test of wireless devices.  Because all waveforms are created and analyzed in an open-source software environment, you can quickly and easily introduce signal impairments, simulate wireless channel conditions, and even apply custom filters.  Combined with the general-purpose Modulation Toolkit, toolkits for GPS, WLAN, and Fixed WiMAX can be used with both mixed signal and RF instrumentation.  As a result, engineers designing GPS, WLAN, and WiMAX devices have access to a new and fully-flexible design tool for these devices.  For more information on simulating communications signals in LabVIEW, please visit http://www.ni.com/wireless/testing_devices.htm. 

Additional Resources

• Download the GPS Toolkit for LabVIEW
• Download the NI Measurement Suite for Fixed WiMAX
• Download the NI WLAN Measurement Suite

 

Reader Comments | Submit a comment »

too much fluff
I had to read through the whole article just to figure out what NI offers to help with this problem; and then read the data sheets for the products to figure out how they might help. I don't think anyone would read the article to begin with if they weren't interested in designing or testing RF systems.
- Dan Lutes, Prophesi Technologies. dlutes@prophesi.com - Feb 8, 2006